This invention relates to an improved resin coated sand for use in shell-molding foundry operations, to eliminate the problems of thermal shock caused when hot metal is cast into the mold. In conventional sand molding operations, a mixture of sand coated with resin binder is placed in a mold, and the heat of the processing steps causes reactions between the binder components to improve the pressed strength of the sand and retain the configuration of the part to be cast.
Upon introduction of the molten metal into the mold, the temperature difference between the molten metal and the mold is great, and the heat of molten metal is transferred to the mold creating thermal shock in the mold, which may create cracks and fissures in the sand mold. The abrupt expansion caused by the temperature differential, destroys the binding action of conventional phenolic binders and cracks and rupture of the mold occur.
The effect of the heat of the molten metal upon the binder is advantageous, since this heat destroys the binder holding the sand or aggregate, and allows, upon cooling, the easy removal of the sand from the cast molded part. The sand is removed by tapping or flogging the molten part to remove the particles. This is known as the shake out property of the mold.
Therefore a resin mixture must be selected that will provide adequate thermal shock protection as well as allow simple removal of the binder-aggregate from the cast item.
A known method to solve the drawback for preventing the molds from cracks has been adopted by means of incorporating cushioning substances into phenolic resins or coated sand obtained therefrom. This method can make the molds flexible as well as free from stress at heating thereof. Said conventional cushioning substances are Vinsol thermoplastic resins, pletroleum resins, rosin, etc. While these substances play a role of cushioning effect in the molds to a certain extent, they have drawbacks in that they emit a disagreeable odor at pouring, due to a thermal decomposition or evaporation thereof. Also, the molds containing such materials are poor in the shake-out property.
After much investigation to overcome said drawbacks, the inventor has found that a shell mold can be made having improved properties and excellent shake out properties if a resin is formed by reacting an aldehyde and a composition containing bisphenol A and bisphenol F in a weight ratio of 98:2 to 80:20.
Bisphenol A is: ##STR1##
Bisphenol F is: ##STR2##